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Abstract:

A measurement probe, for a co-ordinate positioning apparatus such as a
machine tool, is described that includes a stylus holder that is
deflectably mounted to a probe housing. One or more sensors are provided
for sensing deflection of the stylus holder relative to the probe
housing. A processor is included for producing a trigger signal when the
deflection sensed by the one or more sensors meets a trigger condition,
such as a deflection threshold. The probe also includes an accelerometer
for measuring acceleration of the measurement probe. The trigger
condition applied by the processor is alterable, during use, in response
to the acceleration measured by the accelerometer. In this manner, false
triggering can be suppressed.

Claims:

1. A measurement probe, comprising;a stylus holder deflectably mounted to
a probe housing,one or more sensors for sensing deflection of the stylus
holder relative to the probe housing,a processor for producing a trigger
signal when the deflection sensed by the one or more sensors meets a
trigger condition, andan accelerometer for measuring acceleration of the
measurement probe,wherein the trigger condition is alterable, during use,
in response to the acceleration measured by the accelerometer.

2. A measurement probe according to claim 1 in which the trigger condition
comprises a deflection threshold, the trigger signal being issued when
the deflection sensed by the one or more sensors exceeds the deflection
threshold, wherein the deflection threshold is alterable, during use, in
response to the acceleration measured by the accelerometer.

3. A measurement probe according to claim 1 comprising a plurality of
sensors, wherein the trigger condition comprises a plurality of
deflection thresholds and the processor separately compares the
deflection sensed by each of the plurality of sensors to one of the
plurality of deflection thresholds.

4. A measurement probe according to claim 1 comprising a plurality of
sensors, wherein the deflections measured by the plurality of sensors are
combined to provide a resultant deflection, wherein the trigger condition
comprises a resultant deflection threshold and the trigger signal is
issued when the resultant deflection exceeds the resultant deflection
threshold.

5. A measurement probe according to claim 1 wherein the trigger condition
comprises a deflection threshold and a filter delay, the trigger signal
being issued when the deflection sensed by the one or more sensors
continuously exceeds the deflection threshold for longer than the filter
delay, wherein the filter delay is alterable, during use, in response to
the acceleration measured by the accelerometer.

6. A measurement probe according to claim 1 wherein the accelerometer
comprises a micro electro-mechanical system (MEMS) accelerometer.

7. A measurement probe according to claim 1 wherein the accelerometer is
formed from three accelerometer components, the three accelerometer
components being arranged to measure acceleration along three mutually
orthogonal axes.

8. A measurement probe according to claim 1 wherein the trigger condition
is alterable in response to the magnitude and/or the direction of the
acceleration measured by the accelerometer.

9. A measurement probe according to claim 8 wherein the processor is
arranged to analyse the acceleration measured by the accelerometer to
determine the type of motion to which the measurement probe is being
subjected, wherein the alteration made to the trigger condition depends
on the type of motion determined by the processor.

10. A measurement probe according to claim 1 comprising a memory for
storing a plurality of preset trigger conditions, wherein the trigger
condition applied by the processor is selected from the plurality of
preset trigger conditions based on the acceleration measured by the
accelerometer.

11. A measurement probe according to claim 10 wherein predetermined
criteria are used by the processor for selecting a trigger condition from
the plurality of trigger conditions.

12. A measurement probe according to claim 1 wherein the processor
comprises a first processor stage and a second processor stage, the first
processor stage producing a preliminary trigger signal when the
deflection sensed by the one or more sensors meets a first trigger
condition, the second processor stage receiving the preliminary trigger
signal and producing a trigger signal on receipt of the preliminary
trigger signal if the acceleration sensed by the accelerometer is below
an acceleration threshold.

13. A measurement probe according to claim 12 wherein, if the acceleration
sensed by the accelerometer is above the acceleration threshold, the
second processor stage is arranged to modify the first trigger condition
applied by the first processor stage and to only issue a trigger signal
if the first processor stage produces a preliminary trigger signal when
the deflection sensed by the one or more sensors meets the modified first
trigger condition.

14. A measurement probe according to claim 1 wherein the one or more
sensors comprise one or more strain gauge sensors.

15. A measurement probe according to claim 1 comprising a wireless
communications module for passing the trigger signal to a remote probe
interface.

16. A method of operating a measurement probe, the measurement probe
comprising a probe body and a deflectable stylus for contacting a
workpiece, the method comprising the steps of;(i) measuring deflection of
the stylus relative to the probe body,(ii) issuing a trigger signal when
the deflection measured in step (i) meets a trigger condition,wherein,
the method further comprises the step of measuring acceleration of the
measurement probe and altering the trigger condition used in step (ii) in
response to the measured acceleration.

Description:

[0001]The present invention relates to measurement probes for use with
co-ordinate positioning apparatus and in particular to touch trigger
measurement probes mountable in the spindle of a machine tool.

[0002]Touch trigger measurement probes for mounting in the spindle of
machine tools are known. A typical measurement probe of this type
comprises a workpiece-contacting stylus that can be deflected relative to
the body or housing of the probe. One or more sensors are provided to
measure deflection of the probe relative to the probe body and a
so-called trigger signal is issued whenever a certain amount of stylus
deflection has occurred to indicate that the stylus has made contact with
a workpiece. This trigger signal is fed to the machine tool controller
which takes a reading of the position of the machine tool spindle at the
time the trigger signal is issued thereby allowing the co-ordinates of
points on the surface of the workpiece to be measured.

[0003]Examples of strain gauge based touch trigger probes are described in
WO2006/120403 and WO2006/100508. The probe comprises a
workpiece-contacting stylus that is attached to the probe body via a
sensor mechanism that comprises three strain gauges. The signals from the
three strain gauges are passed to a processor which combines and analyses
those signals and produces a trigger signal whenever the deflection of
the workpiece-contacting stylus exceeds a predetermined deflection
threshold or limit.

[0004]Selecting an appropriate deflection threshold is key to ensuring
reliable touch trigger measurement probe operation. If the deflection
threshold is set too low, machine vibrations or movement of the probe
will induce enough stylus deflection to exceed the threshold even in the
absence of workpiece contact; this is typically termed "false
triggering". Conversely, using a high predetermined deflection threshold
reduces the susceptibility to false triggering but increases the amount
of stylus deflection or pre-travel that is required after initial stylus
contact and before the trigger signal is issued. This increased
pre-travel can reduce measurement accuracy in various ways; for example,
errors may arise due to stylus slippage.

[0005]To help prevent false triggering, it is also known to provide a
so-called filter delay so that a trigger signal is only issued by the
probe when stylus deflection continuously exceeds the deflection
threshold for a predetermined amount of time. Introducing a filter delay
can reduce false triggering by ensuring that any transient deflections
(e.g. from machine vibrations or rapid probe movements) do not result in
issuance of a trigger signal.

[0006]It is also known that measurement probes for machine tools often
communicate trigger signals wirelessly to a probe interface that in turn
passes the trigger signal to an input of the coordinate positioning
apparatus. Providing a centrifugal switch or other mechanism to turn on
the measurement sensors in a wireless measurement probe when it is wished
to acquire measurements has also been described previously. For example,
WO 2004/090467 describes a touch trigger probe that automatically
switches itself on when a certain characteristic motion (e.g. rotation of
the probe) is sensed by an accelerometer.

[0007]According to a first aspect of the invention, a measurement probe
comprises; a stylus holder deflectably mounted to a probe housing, one or
more sensors for sensing deflection of the stylus holder relative to the
probe housing, a processor for producing a trigger signal when the
deflection sensed by the one or more sensors meets a trigger condition,
and an accelerometer for measuring acceleration of the measurement probe,
wherein the trigger condition is alterable, during use, in response to
the acceleration measured by the accelerometer.

[0008]The present invention thus provides a measurement probe having a
stylus holder for holding a workpiece contacting stylus that is
deflectable relative to a probe housing. One or more sensors (e.g. strain
gauge, piezo-electric, optical or capacitive sensors) are also provided
to measure deflection of the stylus. The measurement probe also includes
a processor that, in use, is arranged to produce a trigger signal
whenever the stylus deflection as measured by the sensors meets a certain
trigger condition. For example, the trigger condition may be met (i.e.
such that a trigger signal is produced) whenever the measured stylus
deflection continuously exceeds a certain deflection threshold for a
certain period of time. In accordance with the present invention, the
trigger condition applied by the processor is alterable during use and in
particular can be altered in response to the acceleration measured by the
accelerometer. Varying the trigger condition in this manner allows the
sensitivity of the measurement probe to be altered during use and in
particular allows the measurement probe to be desensitised when the
acceleration it is experiencing is of a certain type (e.g. rotation,
vibration, linear acceleration etc) and/or exceeds a certain level.

[0009]A measurement probe of the present invention thus has the advantage,
compared with prior art devices, that it can be both sensitive when being
used to acquire touch trigger measurement data and relatively insensitive
when being subjected to the prolonged accelerations that can be
associated with moving the measurement probe around in the machine
environment between measurements or during tool change operations.
Altering the trigger condition during use in accordance with the present
invention allows acceleration induced deflections that would otherwise
result in issuance of a false trigger to be ignored. In particular, this
improved performance can be achieved without permanently using a high
deflection threshold setting that would also reduce probe sensitivity
during measurement acquisition. The measurement probe of the present
invention can thus provide a touch trigger probe measurement system that
can acquire measurements of the position of points on the surface of an
object with an improved level of confidence compared with prior art
systems.

[0010]The deflection sensed by the one or more sensors may be compared to
many different types of trigger condition by the processor.
Advantageously, the trigger condition comprises a deflection threshold,
the trigger signal being issued when the deflection sensed by the one or
more sensors exceeds the deflection threshold. The deflection threshold
may conveniently be alterable, during use, in response to the
acceleration measured by the accelerometer. For example, the deflection
threshold may be raised in response to an acceleration being measured by
the accelerometer and lowered when such an acceleration is no longer
present.

[0011]If a plurality of sensors are provided, the trigger condition may
comprise a plurality of deflection thresholds and the deflection measured
by each sensor may be separately compared to one of the plurality of
deflection thresholds. The deflection threshold used for each sensor may
be the same or different. Conveniently, each deflection threshold is
alterable, during use, in response to the acceleration measured by the
accelerometer. Each deflection threshold may be altered in a similar, or
different, manner to the other deflection thresholds in response to
measured acceleration. In such an example, the trigger condition may be
met when the deflection sensed by one (or a subset) of the sensors
exceeds the relevant deflection threshold. In other words, a
"first-past-the-post" trigger condition may be provided in which a
trigger signal is issued by the processor when the deflection measured by
one sensor exceeds its threshold.

[0012]Alternatively, the deflections measured by a plurality of sensors
may be combined (e.g. by the processor) to provide a resultant
deflection. The trigger condition may then comprise a resultant
deflection threshold; the trigger signal being issued when the resultant
deflection exceeds the resultant deflection threshold. The deflection
measured by each sensor may be combined to provide a resultant deflection
in a variety of ways; for example, a rectify-and-sum or a sum-of-squares
technique may be used to combine deflection measurements from the
plurality of sensors. Advantageously, deflection signals from a plurality
of sensors may be combined using the technique described in
WO2006/120403, the contents of which are incorporated herein by
reference. The resultant deflection threshold may be alterable, during
use, in response to the acceleration measured by the accelerometer.

[0013]It should also be noted that the actual amount of stylus deflection
(e.g. in microns) need not be calculated. All that is necessary is that
the one or more sensors generate one or more signals that vary in
relation to the amount of stylus deflection. For example, a sensor may
provide a sensor signal of a voltage that is proportional to the amount
of stylus deflection or a number of such sensor signals may be combined
to provide a resultant stylus deflection voltage signal. The trigger
condition may then comprise a deflection threshold in the form of a
voltage threshold; if the voltage of a sensor signal or resultant signal
exceeds the voltage threshold a trigger signal is produced. In such an
example, the voltage threshold could be raised and lowered in response to
the measured probe acceleration.

[0014]Advantageously, the trigger condition comprises a deflection
threshold and a filter delay, the trigger signal being issued when the
deflection sensed by the one or more sensors continuously exceeds the
deflection threshold for longer than the filter delay. The filter delay
is conveniently alterable, during use, in response to the acceleration
measured by the accelerometer. As mentioned above, the deflection
threshold may also be alterable during use.

[0015]The measurement probe may include any known type of accelerometer.
Conveniently, the accelerometer comprises a micro electro-mechanical
system (MEMS) accelerometer. Preferably, the accelerometer is formed from
three accelerometer components. The three accelerometer components may
conveniently be arranged to measure acceleration along three mutually
orthogonal axes. In this manner, the different types of acceleration
(e.g. rotation, linear motion etc) to which the probe is subjected can be
determined. Advantageously, one of the axes along which acceleration is
measured is substantially coincident with the longitudinal probe axis or
the long axis of the probe stylus.

[0016]The trigger condition applied by the processor is advantageously
alterable in response to the magnitude of the acceleration measured by
the accelerometer. For example, the trigger condition may be altered
(e.g. a deflection threshold and/or filter delay increased) in proportion
to the magnitude of acceleration measured by the accelerometer. The
trigger condition applied by the processor may also be alterable in
response to the direction of the acceleration measured by the
accelerometer. For example, the trigger condition may be changed when a
certain direction of acceleration is measured. The trigger condition may
also be alterable in response to both the magnitude and direction of the
acceleration measured by the accelerometer. For example, the trigger
condition may be altered by an amount that is proportional to the
acceleration in a certain direction or by a factor that arises from the
combination of the measured magnitude of acceleration in a plurality of
different directions.

[0017]The trigger condition may conveniently be altered dependent on the
type of acceleration that occurs. Advantageously, the processor is
arranged to analyse the acceleration measured by the accelerometer to
determine the type of motion to which the measurement probe is being
subjected. The processor may thus be arranged to differentiate between
the various different types of acceleration that might be expected, such
as probe rotation, linear movement of the probe, vibration or mechanical
shocks applied to the probe etc. Conveniently, the alteration made to the
trigger condition depends on the type of motion determined by the
processor. For example, the trigger condition may be altered only in
response to certain types of motion or the alteration to the trigger
condition may be different for different types of motion. For example,
the alteration to the trigger condition may be greater for accelerations
perpendicular to the longitudinal or z-axis than for acceleration along
the z-axis. Rotary accelerations or vibrations may also be arranged to
produce a different change to the trigger condition than linear
accelerations of the same magnitude.

[0018]The trigger condition may be continuously or incrementally variable
in relation to the acceleration measured by the accelerometer. The
trigger condition may conveniently be altered between a plurality of
trigger conditions. Preferably, the trigger condition is altered, but
triggering is not totally suppressed, in response to the acceleration
measured by the accelerometer. Advantageously, the trigger condition may
be selected from a stored set of previously determined trigger
conditions. The probe may thus conveniently comprise a memory for storing
a plurality of preset trigger conditions, wherein the trigger condition
applied by the processor (i.e. the trigger condition against which the
deflection sensed by the one or more sensors is compared) is selected
from the plurality of preset trigger conditions based on the acceleration
measured by the accelerometer. Furthermore, predetermined criteria may be
used by the processor for selecting a trigger condition from the
plurality of trigger conditions.

[0019]In other words, multiple possible trigger conditions may be stored
in a memory (e.g. an electronic memory) within the measurement probe and
one of these trigger conditions may then be chosen for use by the
processor based on a set of predetermined criteria. For example, the
processor may compare the deflection measured by the one or more sensors
to a first trigger condition in the absence of any significant
acceleration. The processor may then use a second trigger condition when
linear acceleration is within a certain range, a third trigger condition
when acceleration due to rotation exceeds a certain limit, a fourth
trigger condition when significant vibrations are present etc. In this
manner, the trigger condition applied by the measurement probe in the
presence of particular types of acceleration are determined in advance.

[0020]In a preferred embodiment, the processor comprises a first processor
stage and a second processor stage. The first processor stage is arranged
to produce a preliminary trigger signal when the deflection sensed by the
one or more sensors meets a first trigger condition. The second processor
stage is arranged to receive the preliminary trigger signal and produce a
trigger signal on receipt of the preliminary trigger signal if the
acceleration sensed by the accelerometer is below an acceleration
threshold. In other words, a trigger signal may be issued based on the
first trigger condition if no (or minimal) probe acceleration is present.
It can thus be seen that, in the absence of any acceleration, a trigger
signal is issued based on comparison of the deflection sensed by the one
or more sensors to the first trigger condition. In other words, the
measurement probe operates as a standard probe in the absence of any
significant acceleration.

[0021]If acceleration above the acceleration threshold is sensed by the
accelerometer, the second processor stage may be arranged to not issue a
trigger signal on receipt of the preliminary trigger signal from the
first processor stage. In other words, the second processor stage may
block issuance of a trigger signal if acceleration above a threshold is
sensed by the accelerometer. In this manner, the processor can thus be
seen to apply a normal or first trigger condition when sensed
acceleration is below an acceleration threshold and a second trigger
condition in which issuance of a trigger signal is completely blocked
when acceleration exceeds the acceleration threshold.

[0022]Advantageously, if the acceleration sensed by the accelerometer is
above the acceleration threshold, the second processor stage is arranged
to modify the first trigger condition applied by the first processor
stage and to only issue a trigger signal if the first processor stage
produces a preliminary trigger signal when the deflection sensed by the
one or more sensors meets the modified first trigger condition. In this
manner, a normal trigger response is provided by using the first trigger
condition until a preliminary trigger signal is generated that indicates
the deflection sensed by the one or more sensors meets that first trigger
condition. If the probe is not undergoing any significant acceleration, a
trigger signal is issued. If the probe has been, or is being, subjected
to acceleration the first trigger condition is modified to take account
of such acceleration and the trigger signal is only issued if the
deflection sensed by the one or more sensors meets the first trigger
condition as modified.

[0023]In this manner, modification of the trigger condition only occurs
when the first trigger condition is met. In other words, the trigger
condition applied by the processor is altered when it appears that a
false trigger event is likely to occur as determined from the measured
probe acceleration. Reducing the measurement probe sensitivity, instead
of totally desensitising the probe, when acceleration is sensed has the
advantage that a trigger signal will still be issued if the probe is
deflected by contacting an object.

[0024]It should be noted that the processor (including any constituent
processor stages) may be provided as analogue and/or digital processing
circuitry as appropriate. For example, the processor may be a bespoke
analogue and/or digital (e.g. hardwired) circuit. The processor may also
be provided using programmable logic, such as a field programmable gate
array (FPGA) or similar. The processor may also be implemented via
software running on a general purpose microprocessor. The processor may
be located outside the probe housing (e.g. in a separate interface).
Conveniently, the processor is located within the probe housing.

[0025]Any appropriate sensor or sensors may be used to sense deflection of
the stylus holder relative to the probe housing. For example,
piezo-electric, optical or capacitance based sensors may be provided.
Conveniently, the one or more sensors comprise one or more strain gauge
sensors. Advantageously, three strain gauge sensors are provided. For
example, a strain gauge arrangement of the type described previously in
WO2006/120403 may be provided.

[0026]The measurement probe may comprise a workpiece contacting stylus
formed integrally with the stylus holder. Advantageously, a stylus may be
releasably attached to the stylus holder using a screw thread attachment
or the like. The measurement probe may have a hardwired link to an
interface or machine tool controller for communicating the trigger
signal. Advantageously, the measurement probe comprises a wireless
communications module for passing the trigger signal to a remote probe
interface. The probe may be battery operated and may be configured for
mounting in the spindle of a machine tool.

[0027]According to a second aspect of the invention, there is provided a
method of operating a measurement probe, the measurement probe comprising
a probe body and a deflectable stylus for contacting a workpiece, the
method comprising the steps of (i) measuring deflection of the stylus
relative to the probe body, (ii) issuing a trigger signal when the
deflection measured in step (i) meets a trigger condition, wherein, the
method further comprises the step of measuring acceleration of the
measurement probe and altering the trigger condition used in step (ii) in
response to the measured acceleration.

[0028]As also described herein, a measurement probe may comprise a stylus
holder deflectably mounted to a probe housing, means for producing a
trigger signal when the stylus is deflected by contact with an object,
and an accelerometer for measuring acceleration of the measurement probe,
wherein the sensitivity of the measurement probe to stylus deflection is
decreased when the measurement probe is subjected to acceleration.

[0029]Also described herein is a measurement probe, comprising; a stylus
holder deflectably mounted to a probe housing, one or more sensors for
sensing deflection of the stylus holder relative to the probe housing, a
processor for producing a trigger signal when the deflection sensed by
the one or more sensors meets a trigger condition, and an accelerometer
for measuring acceleration of the measurement probe, wherein the trigger
signal produced by the processor is only output by the measurement probe
when the acceleration measured by the accelerometer is below an
acceleration threshold. In other words, trigger signal issuance is
blocked when the acceleration measured by the accelerometer exceeds the
acceleration threshold. The trigger condition used by the processor may
be alterable during use.

[0030]The invention will now be described, by way of example only, with
reference to the accompanying drawings in which;

[0031]FIG. 1 illustrates a measurement probe of the present invention,

[0033]FIGS. 3a to 3b illustrate altering the trigger condition during
operation, and

[0034]FIG. 4 illustrates a processor of the present invention in more
detail.

[0035]Referring to FIG. 1, a touch trigger measurement probe 2 is
illustrated having a probe housing or body 4 attached to the rotatable
spindle 6 of a machine tool. The spindle 6 is attached to a machine head
(not shown) that can be moved about the machine envelope along three
mutually orthogonal (x,y,z) axes. Movement of the spindle is controlled
by a computer numerical controller 8. The position of the spindle 6 is
also measured by position encoders (not shown) and such positional
information is provided to the CNC 8.

[0036]The measurement probe 2 has a stylus holder 10 to which a stylus 12
is attached by a screw thread connection. The stylus 12 comprises a stem
14 that extends along an longitudinal axis 16 and is terminated by a
stylus tip or ball 18 for contacting an object (e.g. a workpiece or
calibration artefact) mounted to the bed of the machine tool.

[0037]The stylus holder 10 is connected to the probe housing 4 via a
strain sensor 20. In this example, the strain sensor 20 comprises three
fairly rigid, radially spaced, spokes each having a strain gauge attached
thereto for sensing the strain in each spoke. The sensed strain thus
provides an indication of the force with which the stylus 12 is being
deflected relative to the probe body 4. More details about the strain
sensor arrangement can be found elsewhere; for example, see WO2006/100508
and WO2006/120403, the contents of which are hereby incorporated herein
by reference.

[0038]The probe 2 also comprises a processor 22 that receives the outputs
of the strain sensor 20. In particular, the processor 22 receives the
three outputs of the three strain gauges in the form of varying voltage
signals caused by resistance changes induced by the applied strain. The
processor is arranged to combine the three strain gauge signals received
from the strain sensor 20 in a known manner to produce a resultant stylus
deflection signal and to also generate a trigger signal when a certain
trigger condition is met. For example, the processor 22 may issue a
trigger signal when the resultant stylus deflection signal has
continuously exceeded a deflection threshold for a certain period of
time; the period of time being commonly termed the filter delay period or
filter delay. As explained in more detail below, the trigger condition is
not fixed and can be varied during use.

[0039]The probe 2 also comprises a wireless (RF) communications module 24
that communicates the trigger signal to a remote probe interface 26 over
a RF link in a known manner. The trigger signal may then be passed to the
NC 8 by the interface 26. In this manner, the co-ordinate position of the
spindle within the machine envelope can be found whenever a trigger
signal is issued by the measurement probe thereby allowing co-ordinate
position data to be established for points on the surface of an object.

[0040]The touch trigger probe 2 also comprises an accelerometer 28. The
accelerometer 28 is a MEMS based accelerometer that measures three
acceleration components along three mutually orthogonal axes. The
accelerometer 28 is orientated to measure a component of acceleration
along the longitudinal axis 16 and along two axes in a plane
perpendicular to the longitudinal axis 16. The output of the
accelerometer is passed to the processor 22 where it is used to adjust
the trigger condition that is applied by the processor when assessing the
resultant stylus deflection signal generated from the signals received
from the strain sensor 20. In particular, the deflection threshold and/or
filter delay may be increased by the processor in response to an increase
in the measured probe acceleration and reduced when such acceleration
declines.

[0041]Referring to FIGS. 2a to 2c, the operation of a processor applying a
single trigger condition is illustrated. In particular, the FIGS. 2a to
2c illustrate a processor that is arranged to issue a trigger signal when
a deflection threshold d1 is continuously exceeded for a time period t1.

[0042]FIG. 2a illustrates the resultant deflection (voltage) signal from
by the strain sensor 20 of the measurement probe as the stylus is brought
into contact with a point on the surface of an object. It can be seen
that the deflection increases as the stylus tip is driven into the
surface. After initial contact, the deflection threshold is breached and
the deflection remains above that threshold for time t1 whereupon a
trigger signal (T) is issued and probe movement is halted. It should be
noted that, in reality, surface bounce and other effects may cause a
deviation from the shape of the curve that is shown in FIG. 2a,
especially in the time period shortly after initial surface contact is
made.

[0043]As can be seen in FIG. 2b, the use of a filter delay can prevent
some false triggering when the probe is moved around the machine tool
between measurements. In particular, FIG. 2b shows the transient increase
in stylus deflection as a measured probe is accelerated from a first
speed (e.g. zero) to a second speed (e.g. the speed used to move the
probe into the vicinity of the workpiece from a safety plane). Although
the deflection briefly exceeds the d1 deflection threshold, it only does
so for a short period of time. In particular, deflection d1 is not
exceeded for more than the filter delay t1 so there is no (false) trigger
signal issued in this instance. A similar transient crossing of the
deflection threshold occurs in the presence of vibrations or mechanical
shocks to the probe.

[0044]FIG. 2c shows a third situation in which a false trigger signal
would issue. In particular, FIG. 2c shows the resultant deflection signal
produced when the measurement probe is rotated in the spindle. It can be
seen that the rotation causes the sensed deflection to exceed the d1
deflection threshold and this threshold is exceeded for a time period
much greater than the filter delay (t1). A (false) trigger signal T would
thus be issued even though no workpiece contact has occurred.

[0045]Prior to the present invention, it should be noted measurement
probes were often configured to use a certain deflection threshold d1 and
filter delay t1 prior to taking any measurements. In particular, an
operator would set the deflection threshold and filter delay to
appropriate values in an attempt to ensure that any expected movements or
vibration of the measurement probe did not result in a trigger signal
being falsely issued. Although increasing the deflection threshold when
taking measurements is known to reduce false triggering, it also
increases the amount of elapsed time or stylus deflection between the
stylus initially contacting a point on the surface of a workpiece and the
deflection threshold being exceeded. This increase in pre-travel can
result in increased stylus slippage and thus reduced measurement
accuracy. For a typical measurement probe, the filter delay t1 would also
be set to be several milliseconds or tens of milliseconds long.

[0046]Use of such a filter delay can remove false triggers due to
vibration or short periods of probe acceleration but has been found to be
unsuitable for distinguishing between longer periods of acceleration
(e.g. due to probe rotation or probe movement along an arced or circular
path) and deflections due to stylus contact with an object. The maximum
length of the filter delay is, in any case, set by the maximum stylus
over-travel that can occur for a given speed before stylus breakage.
Extension of the filter delay typically can not provide a way of removing
false triggers due to prolonged periods (e.g. of the order of several
tenths of a second) of probe acceleration.

[0047]The present invention allows the trigger condition (e.g. the
deflection threshold and/or the filter delay) to be altered during use.
In particular, the use of probe acceleration data from the accelerometer
contained with the measurement probe is used to dynamically adjust the
trigger condition during use. In other words, prolonged periods of
acceleration can be distinguished from stylus deflection due to workpiece
contact by measuring probe acceleration.

[0048]Referring to FIG. 3, a plot of the sensed resultant deflection as a
function of time is illustrated as a measurement probe is rotated by the
spindle of a machine tool. The deflection threshold being used by the
processor at a particular time is shown in FIG. 3 as a solid line and the
filter delay is kept constant at t1.

[0049]It can be seen that the measured deflection increases rapidly as
probe rotation begins and soon exceeds a first deflection threshold d1.
However, the accelerometer within the probe senses that rotary motion is
underway and increases the deflection threshold from d1 to d2. The new
deflection threshold d2 is higher than the sensed deflection and hence no
(false) trigger signal issues. The slowing of the rotary motion is also
picked up by the accelerometer and the applied deflection threshold is
thus reduced back to d1 as the probe rotation stops. The deflection
threshold d2 is thus applied only during a time period (between points A
and B of FIG. 3) when there is probe rotation and the lower deflection
threshold d1 is applied before and after this time (e.g. during
measurements). In this manner, the measurement probe can be made
insensitive to rotational motion without having to set a higher
deflection threshold that is also used whilst taking measurements.

[0050]It should be noted that although two discrete deflection thresholds
d1 and d2 are shown in FIG. 3, any number of thresholds may be used. For
example, multiple thresholds may be provided and a required threshold
selected based on the magnitude of the measured acceleration at that
particular point in time. The threshold may also be continuously or
incrementally variable in response to the measured acceleration.
Furthermore, the filter delay may also or alternatively be varied in
response to the measured acceleration. The processor may also be arranged
to analyse the outputs of the accelerometer to determine the type of
motion present; the deflection threshold or filter delay may then be set
based on the type and/or magnitude of such motion. The probe may include
a memory to store a look-up table of trigger conditions to be applied in
the presence of certain types and/or magnitudes of acceleration or the
trigger condition may be calculated as required using a set of
predetermined rules.

[0051]The trigger condition may be varied to desensitise the measurement
probe whenever a certain level of acceleration is measured by the
accelerometer. Alternatively, the probe may be desensitised only in the
presence of an acceleration that is found to cause a certain trigger
condition to be met.

[0052]Referring to FIG. 4, an embodiment of the processor of the present
invention is illustrated in more detail. In particular, a processor unit
40 is illustrated that comprises a first processor stage 42 and a second
processor stage 44. The processor unit 40 in this example is an
application specific integrated circuit (ASIC), but many other types of
processor may alternatively be used.

[0053]The first processor stage 42 is arranged to receive a resultant
probe deflection signal 46 from the strain sensor 20 of the measurement
probe. The first processor stage monitors the deflection signal 46 and
issues a preliminary trigger signal 48 when the deflection signal 46
meets a first trigger condition. In this example, the first trigger
condition is met when the deflection signal continuously exceeds a first
deflection threshold d1 for a time t1.

[0054]The second processor stage 44 is arranged to receive the preliminary
trigger signal 48 and also monitors an acceleration signal 50 from the
accelerometer 28. If a preliminary trigger signal 48 is received, the
second processor issues a trigger signal 52 to indicate that workpiece
contact has occurred, but only if no acceleration has been sensed
immediately prior to receipt of the preliminary trigger signal.

[0055]If any acceleration has been sensed, it is possible that the
preliminary trigger signal was produced not because of workpiece contact
but because of a stylus deflection induced by that acceleration. In such
a case, the second processor stage 44 does not issue a trigger signal but
instead passes an instruction signal 54 to the first processor stage 42
that modifies the first trigger condition. This modification may be based
on the magnitude and/or type of acceleration that is sensed.

[0056]The first processor stage then applies the modified trigger
condition to the received deflection signal 46 and continues to issue a
preliminary trigger signal only if the first trigger condition as
modified is met. If the preliminary trigger signal is still issued even
after modification of the first trigger condition, the second processor
stage issues a trigger signal 52. If the preliminary trigger signal is
suppressed following modification of the first trigger condition, it can
be safely assumed that no stylus contact has occurred and no trigger
signal will then be issued by the measurement probe. Once the second
processor stage 44 senses that acceleration has reduced, it may instruct
the first processor stage 42 to apply the (unmodified) first trigger
condition again.

[0057]The two stage process mentioned above has the advantage that the
trigger condition is only adjusted if a preliminary (possibly "false")
trigger signal is generated (e.g. due to the acceleration). In this way,
the first trigger condition alone (e.g. the selected deflection threshold
and filter delay) can be used to suppress certain acceleration induced
deflections and the probe is only desensitised by adjusting the first
trigger condition if this proves inadequate.

[0058]It should be remembered that the specific embodiments described
above are merely examples and that the skilled person would appreciate
the numerous ways in which the present invention could be implemented.